Dried oyster mushrooms are often substituted for tree ear or wood ear mushrooms, but my guess is that if you can't find one locally you're not going to find the other. Tree ears, at least in my way of thinking, don't have a great deal of flavor on their own, but they readily absorb other flavors so are particularly good with saucy dishes. They also have an interesting texture; sort of silky, rather glutinous--but in a good way. Anyway, in terms of your recipe, I'd just double the number of black mushrooms and forget abour the tree ears. You'll lose something in texture, perhaps, but not much in the way of flavor.

Wood/Tree ears also give the dish a bit of rubbery crunchiness. Does that make sense? It's akin to the "bite" of a good sausage or piece of tendon in your meat. It pops when you bite into it, but isn't exactly hard to chew on.

That's a photo of wood's ear mushrooms. They look like that fresh, or you can find them dried in Chinese markets. I've seen them dried in big bags, or dehydrated by machine so that they're compacted into matchbook sized packets. I crumble a corner of the matchbook into a bowl of water and 20 minutes later I have a big bowl of mushrooms. It's really incredible how much water they can soak up!

I love your description, Pei. "Rubbery crunchiness." That's just perfect. And I'd never seen, even in a photograph, fresh wood ears before. Fascinating. I've only found them dried in bags, never fresh or dehydrated.